All consumer color display devices are calibrated so that when the values of color channels Red (R)=Green (G)=Blue (B), the color is displayed at a standard “white point” chromaticity, mostly D65 or D50 according to the International Commission on Illumination (abbreviated as CIE) standard. Digital color cameras using complementary metal-oxide semiconductor (CMOS) or charge-coupled device (CCD) sensors have different sensitivities for RGB channels, resulting in raw images with some color cast (e.g., greenish). Furthermore, the color of an object varies as a function of the color of the light source (e.g., tungsten light or daylight), and the mutual reflection from ambient objects. Therefore, it is often necessary to adjust the “white point” of a raw image before one can process and display the image in proper color reproduction. This white point adjustment is called white balance (WB), and it is typically performed by applying proper gains to the color channels so that neutral objects (such as black, gray, and white) in the image are rendered with approximately equal R, G, B values. In digital cameras, the white point can be manually or automatically adjusted. Automatic white balance (AWB) is thus an important operation in color imaging applications.
Most conventional AWB algorithms rely on some physical features (such as the color gamut) and statistical properties (such as the average color distribution) of natural scenes. The conventional AWB algorithms, which are sensitive to the statistics of the scene contents, often encounter one or more of the following difficulties: 1) a dominant color biases the results, 2) the estimate has a high probability to be wrong when there is no neutral color in the image, 3) inaccurate camera calibration can cause the scene statistics to be different from the statistics used by the camera, 4) a large set of training samples with ground truth may be required to build up reliable statistics, and 5) the algorithm performance may be affected by unit-to-unit variations in the mass production of cameras. Therefore, it is highly desirable to develop AWB techniques that are more robust and relatively insensitive to scene contents.